Process for producing β-form Si3 N4

ABSTRACT

Disclosed is a process for producing  beta -form Si3N4 comprising firing amorphous or  alpha -form Si3N4 of high chemical purity in a non-oxidizing atmosphere under an elevated pressure at a temperature of at least about 1600 DEG  C., as well as a process for producing an article comprising such  beta -form Si3N4.

This application is a continuation, of application Ser. No. 834,818, filed Feb. 28, 1986, now abandoned.

BACKGROUND OF THE INVENTION

This invention concerns a process for producing β-form Si₃ N₃ (hereinafter simply referred to as β-Si₃ N₄) from amorphous or α-form Si₃ N₄ (hereinafter simply referred to as α-Si₃ N₄), as well as a process for producing a Si₃ N₄ article containing β-Si₃ N₄.

Amorphous Si₃ N₄ and crystalline α- or β-Si₃ N₄ are known. Among the three types of Si₃ N₄, β-Si₃ N₄ is considered to have the highest corrosion resistance against molten silicon or the like.

Conventionally in order to prepare β-Si₃ N₄ amorphous or α-Si₃ N₄ having low purity and containing metal oxides such as Y₂ O₃ or metal nitrides such as TiN in amounts of about 5 to 20% by weight has been heat treated at a temperature from 1500 to 1700° C. Accordingly, β-Si₃ N₄ produced by the conventional process contains a relatively large amount of impurities. Reaction sintered Si₃ N₄ is also known as β-Si₃ N₄ having a relatively high chemical purity, but the reaction sintered Si₃ N₄ also contains not less than 0.5% by weight of impurities such as A1, C and O (in total), due to the impurities in the starting Si₃ N₄ material or source Si₃ N₄ material. On the other hand, the CVD (Chemical Vapor Deposition) process is also known as a process for producing Si₃ N₄ having high purity. However, the Si₃ N₄ having high purity produced up until now by the CVD process is amorphous or α-form Si₃ N₄.

No process is yet known for changing the Si₃ N₄ produced by the CVD process into β-form while keeping the purity of the Si₃ N₄ high.

SUMMARY OF THE INVENTION

In view of the foregoing, an object of this invention is to provide a process for producing β-Si₃ N₄ having high chemical purity.

Another object of this invention is to provide a process for producing an article containing β-Si₃ N₄ and comprising Si₃ N₄ having high chemical purity.

In accomplishing these objects, there has been provided according to one aspect of this invention, a process for producing β-Si₃ N₄ comprising the step of firing Si₃ N₄ having high chemical purity in a nonoxidizing atmosphere under an elevated pressure at a temperature of at least about 1600° C.

There has also been provided according to this invention a process for producing a Si₃ N₄ article containing β-Si₃ N₄, comprising the steps of providing an article comprising Si₃ N₄ having high chemical purity and firing said article in a non-oxidizing atmosphere under an elevated pressure at a temperature of at least about 1600° C.

Further objects, features and advantages of this invention will become apparent from the description of preferred embodiments which follows when considered together with the attached figures of drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic view of a furnace for producing amorphous or α-Si₃ N₄ by the CVD process;

FIG. 2 is an a schematic view of a furnace for burning out a graphite plate on which amorphous or α-Si₃ N₄ is deposited;

FIG. 3 is a schematic view of a heat treating furnace for producng β-Si₃ N₄ from amorphous or α-Si₃ N₄ ; and

FIG. 4 is a side view of a graphite substrate for preparing a crucible comprised of β-Si₃ N₄.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a preferred embodiment for producing or synthesizing β-Si₃ N₄ according to this invention, amorphous or α-Si₃ N₃ having high chemical purity is prepared by the CVD process. In the case of preparing amorphous or α-Si₃ N₄ by the CVD process, either the reduced pressure method or normal pressure method may be used. As the reaction gases, combinations of SiCl₄ and NH₃, SiH₄ and NH₃, SiH₂ Cl₂ and NH₃, or any other desired combinations may be used, provided that they are highly chemically pure. The temperature in the reaction chamber is preferably about from 700 to 1300° C. but, if desired, it maybe lower than 700° C. or higher than 1300° C.

A substrate body or substrate base, on which amorphous or α-Si₃ N₄ is deposited by the CVD process, is composed of graphite material or material containing graphite. Silicon, quartz glass, alumina, or the like may be used instead of graphite. The shape of the substrate body or the substrate base is chosen, for example, in view of the ease of the deposition of Si₃ N₄ by the CVD process and, optionally, in view of the ease of removing the substrate body or the substrate base. The substrate body or the substrate base may be formed into such a shape that Si₃ N₄ can be deposited to a shape that embodies the shape of an article.

According to this invention, the gas for the non-oxidizing atmosphere preferably contains N₂ or NH₃, at a partial pressure of at least about 2 atm. When the gas for the non-oxidizing atmosphere does not contain gases having an N ingredient such as N₂ or NH₃, Si₃ N₄ may possibly be decomposed in the firing. The gas for the non-oxidizing atmosphere may contain rare gases such as argon and helium in addition to N₂ or NH₃. In this case, N₂ or NH₃ preferably has a partial pressure of at least 2 atm.

The gas for the non-oxidizing atmosphere may consist essentially of N₂, NH₃ or a mixture of N₂ and NH₃. In the case where the atmospheric gas comprises essentially N₂, it is preferred that the atmospheric gas in the firing is at a pressure of at least about 2 atm and at a temperature of at least about 1600° C. In the case where the atmospheric gas consists essentially of NH₃, the atmospheric gas is preferably at a pressure of at least about 3 atm and at a temperature of at least about 1600° C. in the firing. In the case where the atmospheric gas comprises essentially a mixture of N₂ and NH₃, the atmospheric gas is preferably at a pressure of at least about 2 atm and at a temperature of at least about 1600° C. in the firing.

According to this invention, the non-oxidizing atmospheric gas is preferably at a pressure of at least about 2 atm and, more preferably, at least about 5 atm. If the pressure of the non-oxidizing atmospheric gas in the firing is not higher than about 2 atm, and even if it is higher than 1 atm, the production rate of forming β-Si₃ N₄ is reduced or lowered.

In the procss for producing β-Si₃ N₄ according to this invention, Si₃ N₄ having high chemical purity is fired in the non-oxidizing atmosphere under elevated pressure at a temperature of at least about 1600° C. If the atmosphere in the firing is not non-oxidizing, there is a possibility that at least a portion of the Si₃ N₄ may be oxidized. If the firing is carried out at a pressure lower than the normal pressure (1 atm), for example, under a reduced pressure of 0.1 atm at 1700° C., Si₃ N₄ may be decomposed into Si and N₂. Also, if the firing temperature is lower than about 1600° C., little production of β-Si₃ N₄ occurs even under the elevated pressure.

In this invention, it is preferred to elevate the firing temperature to about 1700-1800° C. in order to enhance the forming or production rate of β-Si₃ N₄. Also, in order to enhance the proportion or ratio of β-Si₃ N₄ produced, it is preferred to increase or elevate the pressure to about 10 atm and increase or elevate the firing temperature to about 1700-1800° C.

In this invntion, the Si₃ N₄ to be fired may be entirely amorphous Si₃ N₄, entirely α-Si₃ N₄ or a mixture of amorphous and α-Si₃ N₄. Also, the ratio of β-Si₃ N₄ in the Si₃ N₄ article is preferably 50% or more. The term "high chemical purity", referring to the purity of Si₃ N₄, means that the content or proportion of the elements, other than those constituting Si₃ N₄, is not more than about 5% by weight.

This invention will be more clearly understood with reference to the following examples:

EXAMPLE 1

At first, using a reaction furnace 1 as shown in FIG. 1, an α-Si₃ N₄ layer 3 was formed on a graphite plate 2 by the CVD process under a reduced pressure. SiCl₄ and NH₃ were used as a reaction gas. SiCl₄ in a 10% mixture in a carrier gas of H₂ (i.e., SiCl₄ /H₂ =1/10 on a molecular basis) was inroduced at a flow rate of 1 1/min from a pipe 4, while NH₃ in a 10% mixture in a carrier gas H₂ (i.e., NH₃ /H₂ =1/10 on a molecular basis) was introduced at a flow rate of 1 1/min from a pipe 5. A pipe 6 was connected to a vacuum pump (not shown) for exhaustion, by which the pressure in a chamber 7 of the reaction furnace 1 was kept at a level of about 20 mmHg. In FIG. 1, a coil 8a for induction heating and a carbon heater 8 associated with the coil 8a are also shown. The graphite plate 2 was used as a substrate. The temperature of the graphite plate 2 in the chamber 7 was maintained at from 700° to 1300° C. by the heater 8. The graphite plate 2 was 100 mm×100 mm×10 mm in size and was fixedly placed on a supporting bed 10 so that the surface 9 having a 100 mm×100 mm area was exposed upwardly. The α-Si₃ N₄ layer 3 was deposited in the form of a plate to a thickness of 3.2 mm on the surface 9 of the graphite plate 2 having a 10 mm×100 mm area over a period of 40 hours.

Then, using an oxidizing furnace 21 as shown in FIG. 2, the graphite plate 2 was burnt out to produce an α-Si₃ N₄ plate 22. Specifically, the graphite plate 2, on which the α-Si₃ N₄ layer 3 had been deposited by the CVD process by using the reaction furnace 1 as shown in FIG. 1, was placed in the chamber 23 of the oxidizing furnace 21, and dry air was introduced from an inlet 24 to the inside of the chamber 23 at a flow rate of 5 1/min and then exhausted from outlet 25. In the oxidizing treatment, the temperature in the chamber 23 was maintained from 600°to 800° C. by a heater 26. In order to burn out the graphite plate 2, oxygen gas may be used instead of dry air. In the case of using oxygen gas, the flow rate thereof may be lower than that of the dry air. In order to remove the graphite plate 2, mechanical means such as cutting or grinding may also be used. Further, depending on the use of the Si₃ N₄ product produced, the graphite plate 2 may be left. On the other hand, in the case where the substrate or base for forming the Si₃ N₄ layer thereon by the CVD process is made of a material different from graphite or carbon, for example, silicon or quartz glass, etching means known in the field of the semiconductor processing art may be used for the removal of the substrate.

Then, the α-Si₃ N₄ plate 22 produced by burning out the graphite plate 2 in the oxidizing furnace 21 shown in FIG. 2 was treated in a heat treating furnace 41 under an elevated pressure as shown in FIG. 3. For treatment in the furnace 41, the α-Si₃ N₄ plate 22 was disposed in a chamber 42 of the heat treating furnace 41, and N₂ gas at 10 atm was introduced from a conduit 43 into the chamber 42. And then, the temperature inside the chamber 42 was elevated to 1750° C. by a heater 45 while closing a gas charge/discharge valve 44. Th epressure of the N₂ gas in the chamber 42 was 10 atm. After the heat treatment under the elevated pressure for 10 hours, 80% by weight of α-Si₃ N₄ constituting the initial or original Si₃ N₄ plate 22 was changed or converted into β-Si₃ N₄ and a Si₃ N₄ plate 46 having a composition of 80% by weight of β -Si₃ N₄ and 20% by weight of α-Si₃ N₄ was produced (hereinafter referred to as sample 1).

TEST 1 (CORROSION RESISTANCE TEST)

Then, the Si₃ N₄ plate produced in accordance with Example 1 (80% β-Si₃ N₄ and 20% α-Si₃ N₄ ; sample 1), α-Si₃ N₄ as it existed before the heat treatment under the elevated pressure as a Comparative Example (sample 2) and a quartz glass or silica glass SiO₂ for use in a crucible employed in a conventional silicon crystal pulling method (sample 3) as another Comparative Example were immersed in a molten silicon contained in a quartz glass crucible at a temperature of 1550° C. for two hours. The extent or the rate of the corrosion for each of the sample plates (each of a same size, 5 mm×100 mm×3 mm in size before the immersion) was measured. As a result, the corrosion rate for each of the samples is shown in Table 1.

                  TABLE 1                                                          ______________________________________                                                              Corrosion                                                 Sample               rate        Corrosion                                     No.     Composition  (/um/hr.)   Resistance                                    ______________________________________                                         1       80% β-Si.sub.3 N.sub.4                                                                  10         good                                          2       α-Si.sub.3 N.sub.4 (CVD)                                                              150         poor                                          3       SiO.sub.2    180         poor                                          ______________________________________                                    

From Table 1, it can be seen that sample 1 of Example 1 comprising 80% by weight of β-Si₃ N₄ and 20% by weight of α-Si₃ N₄ according to this invention has more excellent corrosion resistance to the molten silicon than that of sample 2 made of α-Si₃ N₄ produced by the CVD process or sample 3 made of conventional quartz glass.

TEST 2 (PURITY)

Impurity contents were examined for sample 1 produced in accordance with Example 1 and a sample 4 comprising β-Si₃ N₄ produced by a known reaction sintering process and having higher purity than that of the sintered Si₃ N₄ product which had generally been produced by using an ordinary sintering agent or aid. The results are shown in Table 2.

                  TABLE 2                                                          ______________________________________                                         Sample           Kind of Impurity (ppm)                                        No.   Composition                                                                               Na     Ca    Fe   Al    Cr   Mn                               ______________________________________                                         1     80% β-Si.sub.3 N.sub.4                                                               less   less  less less  less less                                   20% α-Si.sub.3 N.sub.4                                                              than   than  than than  than than                                              10     10    10   10    10   10                               4     β-Si.sub.3 N.sub.4                                                                   40     1100  3900 2300  400  80                                     (reaction                                                                sintering)                                                                     ______________________________________                                    

As is apparent from Table 2, the Si₃ N₄ of sample 1 produced in accordance with this invention has a considerably lower impurity content than that of sample 4 obtained by the conventional reaction sintering. Furthermore, as for metal impurities other than those described in Table 2, sample 1 contained the impurities at much lower level than the sample 4.

From the results of the tests 1 and 2, sample 1 produced in accordance with Example 1 is superior to α-Si₃ N₄ and a quartz glass in terms of corrosion resistance to the molten silicon and also superior to the conventional reaction sintered β-Si₃ N₄ in terms of the impurity content. For instance, sample 1 is suited as a material to prepare a crucible for containing or accommodating molten silicon therein for producing single crystal or polycrystalline silicon or amorphous silicon by the pulling-up method. In the case of preparing or forming the crucible, the graphite material used as the substrate may previously be shaped in the form or configuration, for example, shown by reference numeral 50 in FIG. 4. Then, amorphous or α-Si₃ N₃ is deposited on the surface 51 of the graphite material 50 by the CVD process as shown by reference numeral 52. The deposited layer 52 substantially forms the shape of the crucible.

The β-Si₃ N₄ or the Si₃ N₄ product produced in accordance with this invention can be applied to various uses other than the crucible, such as turbine blades or the like.

EXAMPLE 2

First, amorphous Si₃ N₄ was prepared by the CVD process according to the same procedures as in Example 1 except that the temperature during the CVD process was set at 1000° C. Then, the graphite plate was burnt out in the same manner as in Example 1. Subsequently, the heat treatment under elevated pressure was carried out in the same manner as in Example 1. The resulting Si₃ N₄ comprised 75% by weight of β-Si₃ N₄ and 25% by weight of α-Si₃ N₄ (sample 5) as shown in Table 3.

Furthermore, the Si₃ _(l) N₄ plates prepared by the CVD process according to the same procedures as in Example 1 were fired under various conditions of heat treatment under elevated pressure for 10 hours to produce samples 6-17 as shown in Table 3.

                                      TABLE 3                                      __________________________________________________________________________     Condition for Heat Treatment Under Elevated Pressure                                                          Resultant                                       Sample                                                                             Atmospheric                                                                           Pressure                                                                              Temperature                                                                           Thickness                                                                            Composition                                     No. gas    (atm)  (°C.)                                                                          (mm)  α (%)                                                                        β (%)                                  __________________________________________________________________________     5   N.sub.2                                                                               10     1750   about 3                                                                              25  75                                          6   N.sub.2                                                                               less than 2                                                                           1750   about 3                                                                              85  15                                          7   N.sub.2                                                                               2      1750   about 3                                                                              35  65                                          8   N.sub.2                                                                               2      1600   about 3                                                                              47  53                                          9   N.sub.2                                                                               2      less than 1600                                                                        about 3                                                                              98  2                                           10  NH.sub.3                                                                              less than 2                                                                           1750   about 3                                                                              81  19                                          11  NH.sub.3                                                                              2      1750   about 3                                                                              37  63                                          12  NH.sub.3                                                                              2      less than 1600                                                                        about 3                                                                              99  1                                           13  NH.sub.3                                                                              10     1750   about 3                                                                              25  75                                          14  N.sub.2 and NH.sub.3                                                                  2      1750   about 3                                                                              35  65                                          15  N.sub.2 and NH.sub.3                                                                  10     1750   about 3                                                                              22  78                                          16  N.sub.2 and Ar                                                                        2      1750   about 3                                                                              42  58                                          17  N.sub.2 and Ar                                                                        10     1750   about 3                                                                              36  64                                          __________________________________________________________________________

The results of the corrosion resistance test carried out in the same manner as in Test 1 of Example 1 for samples 5-17 are shown in Table 4. The results of the purity evaluation for each of the samples 5-17 were the same as those for sample 1 of Example 1.

As is apparent from Tables 3 and 4, as regards the relationship between the conditions for the heat treatment under elevated pressure and the production rate or proportion of β-Si₃ N₃, the ratio or proportion of β-Si₃ N₄ is increased as the processing pressure is raised and as the heating temperature is raised. However, if the treatment is carried out at a temperature higher than 1850° C., there is a possibility that Si₃ N₄ may tend to be decomposed. Also, as regards the relationship between the ratio or proportion of β-Si₃ N₄ and the corrosion resistance, the corrosion resistance is remarkably improved when the ratio of β-Si₃ N₄ exceeds 50% by weight.

                  TABLE 4                                                          ______________________________________                                         Sample Composition (%) Corrosion   Corrosion                                   No.    α-Si.sub.3 N.sub.4                                                                 β-Si.sub.3 N.sub.4                                                                  Rate (/μm/hr.)                                                                        Resistance                                ______________________________________                                          5     25        75        15        good                                       6     85        15        105       poor                                       7     35        65        17        good                                       8     47        53        33        good                                       9     98         2        110       poor                                      10     81        19        100       poor                                      11     47        63        19        good                                      12     99         1        120       poor                                      13     25        75        17        good                                      14     35        65        18        good                                      15     22        78        13        good                                      16     42        58        35        good                                      17     36        64        23        good                                      ______________________________________                                     

What is claimed is:
 1. A process for producing a β-form Si₃ N₄ article, comprising the steps of:depositing a Si₃ N₄ starting material comprising less than about 0.5% by weight of impurities on a substrate by chemical vapor deposition; firing the Si₃ N₄ deposit in a non-oxidizing atmosphere consisting essentially of one of N₂, NH₃ and mixtures thereof under an elevated pressure of at least about 2 atm at a temperature of at least about 1700° C. so as to obtain a Si₃ N₄ article comprising at least about 60% by weight of β-form Si₃ N₄ ; and removing the substrate after the deposition step.
 2. A process according to claim 1, wherein said Si₃ N₄ starting material comprises α-form Si₃ N₄.
 3. A process according to claim 1, wherein said Si₃ N₄ starting material comprises amorphus Si₃ N₄.
 4. A process according to claim 1, wherein said non-oxidizing atmosphere comprises N₂ gas.
 5. A process according to claim 1, wherein said non-oxidizing atmosphere comprises NH₃ gas.
 6. A process according to claim 1, wherein said non-oxidizing atmosphere comprises a mixture of N₂ gas and NH₃ gas.
 7. A process according to claim 1, wherein said elevated pressure in the firing is at least about 5 atm.
 8. A process according to claim 1, wherein said temperature in the firing is less than about 1850° C.
 9. A process according to claim 1, wherein said substrate comprises carbon.
 10. A process according to claim 1, wherein said substrate comprises quartz glass.
 11. A process according to claim 1, wherein said substrate comprises silicon.
 12. A process according to claim 1, wherein said Si₃ N₄ article comprises less than about 0.5% by weight of impurities.
 13. A process according to claim 8, wherein the firing step is maintained at a pressure of about 10 atm.
 14. A process according to claim 1, wherein said impurities comprise Na, Ca, Fe, Al, Cr and Mn.
 15. A process according to claim 14, wherein said impurities comprise an amount equal to less than 10 ppm each.
 16. A process according to claim 15, wherein said Si₃ N₄ article comprises about 80% by weight of β-form Si₃ N₄.
 17. A process for producing a β-form Si₃ N₄ article, comprising the steps of:depositing a Si₃ N₄ starting material comprising less than about 0.5% by weight of impurities on a substrate selected from the group consisting of carbon, quartz glass and silicon by chemical vapor deposition; firing the Si₃ N₄ deposit in a non-oxidizing atmosphere consisting essentially of one of N₂, NH₃ and mixtures thereof under an elevated pressure of at least about 2 atm at a temperature of at least about 1700° C. so as to obtain a Si₃ N₄ article comprising at least about 60% by weight of β-form Si₃ N₄ ; and removing the substrate after the deposition step.
 18. A process according to claim 11, wherein the starting material comprises amorphous or a-form Si₃ N₄ prepared by chemical vapor deposition. 